1. Field of the Invention
The present invention is directed to imaging devices whose pixels are separately addressable, and more particularly to the elimination of artifacts in light valves and liquid crystal displays caused by post spacers and other departures from nominal operation.
2. Description of Prior Art
A typical light valve comprises a thin layer of liquid crystals interposed between two glass plates coated with thin layers of transparent conductors such as indium tin oxide and polymer alignment films. Light valves and liquid crystal displays generate images by modulating the amount of transmission, reflection and scattering of an incoming light source.
In a typical liquid crystal display polarizers are on the top glass plate as well as on the bottom glass plate. On reflective light valves, the bottom glass plate is replaced by a reflective mirror. The pixels are individually addressed by a matrix of conducting electrodes and typically driven by transistors fabricated on a silicon or amorphous silicon substrate.
The liquid crystals' orientation above each pixel can be changed, based on its optical anisotropy, by driving the transistor at different voltages. By changing the liquid crystals' orientation, incoming polarized light from one polarizer can be rotated, thereby determining the amount of light transmitted through the other polarizer. The amount of reflected or scattered light in a reflective light valve or a liquid crystal display is determined by the rotation of the polarized light and the reflection of the mirror layer.
During the assembly of light valves and liquid crystal displays it is critically important to maintain uniformity in the gap distance occupied by the liquid crystals. It is also critically important to prevent contamination of the liquid crystal layer. In the practical manufacturing of light valves and liquid crystal displays, the cost can be significantly reduced if the fabrication tolerances permit a small number of defective pixels whose operation is perceptibly different from nominal operating conditions. There are many reasons for these defects, such as shorts and opens in the conductor electrodes, transistors that operate away from nominal operating conditions, non-uniform charge retention, contamination during processing, defective mask steps during photo-lithography and non-uniform cell gap control.
In liquid crystal displays or projection systems it is essential to achieve a good cell gap control, even as simultaneously attempts are made to reduce voltage to the minimum level consistent with the driver technology employed. To obtain optimal response with many liquid crystal modes, such as twisted nematic for example, accurate control of cell gap is essential in order that uniform grey scale, fast switching speeds, and high contrast be maintained over the cell.
In modern display technologies, the liquid crystals operate in modes that require strict tolerance and uniformity in the cell gap over the full area of the display. One solution for maintaining a good cell gap between the glass layers is to use interposing post spacers in the pixel array region. Post spacers are sometimes referred to just as spacers or posts. Fortunately, when light valves based on silicon technology are used, it becomes fairly straight forward to place these spacers in the boundaries between the pixel mirrors within the cell. These boundaries are dark, so the spacers are not easily seen directly. However, the post still perturbs the liquid crystal alignment above the mirrors of the surrounding pixels, causing changes in the polarization efficiency of the liquid crystals in the region around the posts. Changes in the polarization efficiency change the luminance and the chrominance in the vicinity of post spacers when the light valve is imaged on the screen. Even with a minimum resolution post element, delineated by modern fine-line lithography, the posts will still show artifacts when imaged.
Shadowing from the rubbing of liquid crystals will make the imaged artifacts still more visible. The shape of the post can be physically modified to reduce post artifacts, but this increases the fabrication difficulty and does not provide a solution for artifact elimination over the full luminance range generated by the display.
These defects detract from the appearance of images and therefore limit the commercial usefulness of a display. What is needed is a way to deploy posts without creating imaged artifacts. This method must be robust in dealing with variability in cell process and changing illumination conditions.